There is need for low cost, rapid, and accurate means for quantitative analysis of whole blood and other body fluids in medical diagnostics.
Some of the methods representative of the current state of the art have been recently reviewed by Bluestein, et al, [Bluestein, B. I., Walczak, I. M., and Chen, S., "Fiber Optic Evanescent Wave Immunosensors for Medial Diagnostics" Tibtech, 8, 161-168, June, 1990].
Biosensors provide rapid response, real time monitoring of the sample/sensor interaction to produce an electronically quantified result. A biomolecular component of the sensor is used to recognize the analyte of interest. These recognition components include molecules such as antibodies, enzymes, lectins, hormones, DNA, and neuro-transmitter receptors. For example, when antibodies are used to recognize the appropriate antigens, the device can be referred to as an immunosensor. Typically, the recognition or binding of antibodies and antigens is a chemical reaction with very high equilibrium association constant. The sensors are not reusable; they are used once and then discarded. Therefore, a suitable device must provide for easy replacement of the sensor portion, and the sensor must be inexpensive and easy to manufacture.
Devices for obtaining a quantitative result from the binding reaction have used techniques such as electrochemical, piezoelectric, capacitance, and optical detection schemes. One type of optical device uses surface plasmon resonance for measurement and is characterized by use of a metallic or metal-like film that interacts with the light in a manner that varies with the angle of light beam incidence. Another type of optical device, often referred to as evanescent wave type, requires the use of a fluorphor such as fluorescein and measures resultant fluorescent radiation. Such sensors typically comprise flat plates or the surface of cylindrical rods as reviewed by Bluestein, et al.
Another class of sensors, known as distal tip sensors, use optical fibers to convey light to the distal end where the reaction is monitored by reflected light form light scattering or fluorescence from fluophors.
Optical fibers have been used in devices to measure the density of protein in blood as described by Minekane in Japanese Patent 56-107419, issued Aug. 25, 1981, for a Densitometer (Application 55-9304). The fiber dipped in the sample is in the shape of a U, with the core exposed at the bent bottom. Light leaks from the exposed core depending on the concentration of protein, and the reduced light intensity that is transmitted is compared to the initial light intensity. It is well known that the leakage of light from an optical fiber depends on the radius of curvature and the index of refraction of the medium adjacent the fiber such as the cladding or the liquid sample in contact with the exposed fiber core. In such devices, which comprise a form of refractometer, the flexible unsupported fiber can have only a short length of the fiber exposed to the liquid while maintaining a fixed curvature. The limited exposed area precludes adaptation of this technique to immunoassay requiring a coating of antibodies on a significant area of the optical fiber. One typical embodiment of the present invention provides a novel way of adapting the principle of leaky fibers to planar integrated optic devices for immunoassay.
The current known technology for fabricating integrated optical devices in the form of channel waveguides is adequate for producing small optical devices by mass production techniques using photolithography and microfabrication. Such technology is applicable to typical embodiments of the present invention directed to accurate, lost-cost, replaceable sensors.
Integrated optical transducers comprising channel waveguides in the form of Mach Zehnder interferometers are known. Johnson (U.S. Pat. No. 4,515,430) discloses such an interferometer for measurement of temperature. Arms of unequal length in the interferometer utilize the temperature dependent coefficient of expansion of waveguides. Fabricius, et al (German Patent DE 381 4844, European Patent 340577) disclose an interferometer comprising a reference arm covered with substrate and an exposed measurement arm for measuring the refractive index of liquids. Means for compensating the effect of variation in source light intensity are not disclosed. The exposed waveguide arm does not contain a coating for reaction with components of the liquid sample as required for immunoassay and as in some typical embodiments of the present invention.
Another type of integrated optic device, the directional coupler, is employed in other typical embodiments of this invention. Such devices have not previously been considered for use in measurement of liquid properties or chemical reactions.